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Employee Future Benefits - Status of Plans (Details) - CAD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
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Disclosure of net defined benefit liability (asset) [line items] |  |  |
Net defined benefit liability (asset) | $ (294) | $ (278) |
Accrued current liabilities |  |  |
Disclosure of net defined benefit liability (asset) [line items] |  |  |
Net defined benefit liability (asset) | (12) | (10) |
Other long-term liabilities |  |  |
Disclosure of net defined benefit liability (asset) [line items] |  |  |
Net defined benefit liability (asset) | (282) | (268) |
Fair value of plan assets |  |  |
Disclosure of net defined benefit liability (asset) [line items] |  |  |
Net defined benefit liability (asset) | 381 | 386 | $ 381
Present value of defined benefit obligation |  |  |
Disclosure of net defined benefit liability (asset) [line items] |  |  |
Net defined benefit liability (asset) | (675) | (664) |
Registered |  |  |
Disclosure of net defined benefit liability (asset) [line items] |  |  |
Net defined benefit liability (asset) | (175) | (170) |
Registered | Accrued current liabilities |  |  |
Disclosure of net defined benefit liability (asset) [line items] |  |  |
Net defined benefit liability (asset) | (5) | (3) |
Registered | Other long-term liabilities |  |  |
Disclosure of net defined benefit liability (asset) [line items] |  |  |
Net defined benefit liability (asset) | (170) | (167) |
Registered | Fair value of plan assets |  |  |
Disclosure of net defined benefit liability (asset) [line items] |  |  |
Net defined benefit liability (asset) | 367 | 373 | 368
Registered | Present value of defined benefit obligation |  |  |
Disclosure of net defined benefit liability (asset) [line items] |  |  |
Net defined benefit liability (asset) | (542) | (543) | (514)
Supplemental Pension Plan |  |  |
Disclosure of net defined benefit liability (asset) [line items] |  |  |
Net defined benefit liability (asset) | (95) | (86) |
Supplemental Pension Plan | Accrued current liabilities |  |  |
Disclosure of net defined benefit liability (asset) [line items] |  |  |
Net defined benefit liability (asset) | (5) | (5) |
Supplemental Pension Plan | Other long-term liabilities |  |  |
Disclosure of net defined benefit liability (asset) [line items] |  |  |
Net defined benefit liability (asset) | (90) | (81) |
Supplemental Pension Plan | Fair value of plan assets |  |  |
Disclosure of net defined benefit liability (asset) [line items] |  |  |
Net defined benefit liability (asset) | 14 | 13 | 13
Supplemental Pension Plan | Present value of defined benefit obligation |  |  |
Disclosure of net defined benefit liability (asset) [line items] |  |  |
Net defined benefit liability (asset) | (109) | (99) | (80)
Other |  |  |
Disclosure of net defined benefit liability (asset) [line items] |  |  |
Net defined benefit liability (asset) | (24) | (22) |
Other | Accrued current liabilities |  |  |
Disclosure of net defined benefit liability (asset) [line items] |  |  |
Net defined benefit liability (asset) | (2) | (2) |
Other | Other long-term liabilities |  |  |
Disclosure of net defined benefit liability (asset) [line items] |  |  |
Net defined benefit liability (asset) | (22) | (20) |
Other | Fair value of plan assets |  |  |
Disclosure of net defined benefit liability (asset) [line items] |  |  |
Net defined benefit liability (asset) | 0 | 0 | 0
Other | Present value of defined benefit obligation |  |  |
Disclosure of net defined benefit liability (asset) [line items] |  |  |
Net defined benefit liability (asset) | $ (24) | $ (22) | $ (25)

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- Definition Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes of the table. + References No definition available. + Details
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- Definition The amount of deficit or surplus in a defined benefit plan, adjusted for any effect of limiting a net defined benefit asset to the asset ceiling. [Refer: Defined benefit plans [member]] + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name IAS -Number 19 -IssueDate 2019-01-01 -Paragraph 140 -Subparagraph a -URI http://eifrs.ifrs.org/eifrs/xifrs-link?type=IAS&num=19&code=ifrs-tx-2019-en-r&anchor=para_140_a&doctype=Standard -URIDate 2019-03-27 + Details
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